This invention relates generally to gas turbine engines and more particularly to purging forward or aft wheel cavities in the turbine sections of such engines.
A high bypass ratio turbofan engine used for powering an aircraft in flight typically includes a fan, a low pressure compressor or booster, a high pressure compressor, a combustor, a high pressure turbine and a low pressure turbine in axial flow relationship. A portion of the air entering the engine passes through the fan, booster and high pressure compressor, being pressurized in succession by each component. The compressed air exiting the high pressure compressor, commonly referred to as the primary or core gas stream, then enters the combustor where the pressurized air is mixed with fuel and burned to provide a high energy gas stream. However, prior to entering the combustor a portion of the primary or core flow is diverted to provide a source of cooling air for various high temperature components, such as those found in the high pressure turbine. After exiting the combustor, the high energy gas stream then expands through the high pressure turbine where energy is extracted to operate the high pressure compressor, which is drivingly connected to the high pressure turbine. The primary gas-stream then enters the low pressure turbine where it is further expanded, with energy extracted to operate the fan and booster, which are drivingly connected to the low pressure turbine. The remainder of the air flow (other than the primary flow) that enters the engine passes through the fan and exits the engine through a system comprising annular ducts and a discharge nozzle, thereby creating a large portion of the engine thrust.
The high pressure turbine typically includes one or two stages, while the low pressure turbine ordinarily has a larger number of stages. Each stage generally includes a rotor and a stator. The rotor comprises a rotor disk that rotates about the centerline axis of the engine and supports a plurality of blades that extend radially into the primary gas stream. The stator includes a row of stationary nozzles that direct the primary gas stream in such a manner that the rotor blades can do work. In a multi-stage turbine, the blades of one stage are located immediately downstream from the nozzles of that stage, and the nozzles of the next stage are located immediately downstream from the prior stage's blades. However, counterrotating engines (i.e., engines in which the high pressure turbine and the low pressure turbine rotate in opposite directions) typically do not have a stage of nozzles located between the last stage high pressure rotor and the first stage low pressure rotor.
Rotating labyrinth seals are commonly used in the high and low pressure turbines for sealing the above-mentioned cooling air from the primary gas stream. A rotating labyrinth seal is made up of a number of thin, tooth-like projections extending radially from a rotating engine part with their free ends disposed in sealing engagement with a stationary engine part or an engine part that is rotating in the opposite direction. However, because the unsealed spaces fore and aft of the rotor disks, commonly referred to as the wheel cavities, are in fluid communication with the primary gas stream, a flow of cooling air into the cavities is necessary to purge the cavities and prevent hot gas ingestion. A failure to maintain adequate purge flow can lead to significantly reduced part life of adjacent components.
Conventional engines rely on leakage through the labyrinth seals and the use of air holes in adjoining engine parts to supply purge air to the wheel cavities. However, the stress concentrations associated with the air holes create the potential for cracking and premature failure of rotating engine parts. Also, the machining necessary to form the air holes will incrementally increase the cost of manufacturing the parts.
Accordingly, there is a need for a means of properly purging the wheel cavities of a turbine section without the use of air holes.